Help me understand house air conditioning

Nakatomi2010 · June 2010

My wife and I recently bought ahouse in October, all throughout winter the A/C worked fine, as it should since it was cool out to begin with.

At this point I'm going to mention we live in Florida, A/C during the winter is not uncommon.

Starting about the beginning of May we began to have some cooling problems in our house, getting worse overtime.

I shall use the following text base diagram to illustrate our house layout.

....................
.....

....
.....|_____|...
...o|_____|...
.....|_____|...
.....

....

The 'o' represents the location of the air conditioner. The sun rises on the right and sets on the left (Sets shning hard against the air conditioner)

There are no homes on either side of us.

Our porblem is that starting at about 1 or 2 in the afternoon when the sun is setting and shining against our air conditioner the temperature of the house rises to about 81 degrees, when the thermostat is set to 77. So we're gaining as much as 4 degrees just by having the sun on that side of the house.

I've called the air conditioner folks out to look at it 4 times (Warranty as house is less than 7 months old) and they all say nothing is wrong, the vents are pushing 75 degrees, while the thermostat says 81. The solution of the company's "Service manager" was to increase the speed of the fan, comparing it to a car's fan which as you set it higher gets it cooler quicker (louder too). The service manager also told us that our problems are related to the sun shining against the system in the afternoon/evening.

So the ulatimate question is this: Is our 4 degree spike in temperature at night really caused by the sun setting and shining against our air conditioner, or do I have a problem that the company who installed it is inept at finding?

Druhim · June 2010

yeah, basic physics dude
the sun is in fact making your house hotter in the afternoon so your AC has to work harder and it seems it's not able to fully keep up

Raneados · June 2010

well it doesn't really get hotter at NIGHT, it gets hotter in the afternoon

which is the hottest part of the day

so yeah, your AC unit just can't keep up with how hot your house gets in the afternoon

A Dabble Of Thelonius · June 2010

Sounds like an undersized unit. Or it's just old and needs to be replaced.

tardcore · June 2010

If that is in fact the problem, then there isn't much you can do outside of buying some fans/getting some blinds to block out the sun during the day.

Druhim · June 2010

or forking out the dough for a more powerful air conditioner

DanMach · June 2010

.........or shade your AC unit.........

SkyCaptain · June 2010

DanMach wrote: »

.........or shade your AC unit.........

Will not help. It's the house that needs the shade, not the AC unit.

@OP - Can you plant some trees to help shade the house in the afternoon?

Midshipman · June 2010

SkyCaptain wrote: »

DanMach wrote: »

.........or shade your AC unit.........

Will not help. It's the house that needs the shade, not the AC unit.

Not true. The hotter the environment that that condensor coils are in, the less efficiently the AC system will be able to operate. Shading the AC box from direct sunlight will help out to some extent.

genejocky · June 2010

You might also want to check the insulation, especially since it is a new house. I had a similar issue with a new house my wife and I bought in Louisiana. Turned out that when they put insulation in the attic (blown cellulose), the guy just stood on the ladder up to the attic and blew the cellulose in a circle just as far as he could reach without moving. I called the contractor back out and he finished the job, made a lot of difference.

Djeet · June 2010

The backside of my house is an unshaded southern exposure. Putting up a radiant barrier in the attic helped keep internal temperature rises down and now the house is quicker to cool by A/C. I tacked this up this with 5/8" staples directly to the rafters. If you don't want to do the work you can hire insulation specialists to spray a radiant barrier to the underside of your roof. I've heard that the tacked up radiant barrier is more effective though.

If you want to DIY you'll want to do this early in the morning before your attic becomes a furnace. You'll also want a scissors or boxcutters and some insulation tape; I also marked the paths of electrical wiring. After I installed mine my attic must've been at least 20 degrees cooler at the height of the afternoon. Is your attic properly ventilated? Make sure no vents are blocked and ideally there should be a fan blowing out a vent so as to help evacuate hot air.

As to the AC you might consider having your ducts checked for tears//leaks and cleaned out. Assuming the heat's already hit there, you may have to wait awhile to get a simple HVAC service call.

Metalbourne · June 2010

this is a simple matter of thermodynamics.

As you probably know, heat moves to where there is more to where there is less.

Also, the greater the difference between the heat pushed out by the air conditioner and the outside temperature, the more efficient the unit.

On a semi-unrelated note, if the service guy doubled the speed of your fan, it will quadruple the amount of pressure it generates, but draw eight times more horsepower. be prepared for higher electricity bills.

I would put a shade over the AC unit. It might not help, but it definately can't hurt.

GungHo · June 2010

I'd put some shading around the unit... like a fence or tall bushes. Leave enough room for you to be able to navigate yourself/a lawn mover around it. You do not want to completely enclose the unit... it needs free airflow out of the top. That will help keep the condenser cooler. You may also want to look at some other parts of your house...

Do you have double-paned, vinyl windows?
Have you looked at getting the windows polarized/screened?
Do you use cellulose insulation... is it spread evenly in the attic and walls (see Genejocky's post)?
Do you have a radiant barrier along your roof (something like HeatBloc 75)?
Is your attic ventilated?
Have you changed your A/C & attic media filters out since the house was completed? Are you using good quality filters? (I recommend 3M Filtrete, 1500 MPR or better.)
Do any of your rooms along that south western exposure have a lot of windows?

If you're worried about your home builder giving you the run around, you can request an independent energy audit. You can also hire an inspector to come out with a temperature gun. There should be a temperature differential of between 16~21F.

In general, I'd advise you to have an inspector come out in a couple of months anyway... have him look at everything (general home inspection). That way, you can get everything settled before your warranty expires.

Ruckus · June 2010

Shading the AC condenser unit may help, I'd say leave at least 4 feet on all sides for airflow. Other than that, I'd say insulation is the way to go.

Enc · June 2010

Midshipman wrote: »

SkyCaptain wrote: »

DanMach wrote: »

.........or shade your AC unit.........

Will not help. It's the house that needs the shade, not the AC unit.

Not true. The hotter the environment that that condensor coils are in, the less efficiently the AC system will be able to operate. Shading the AC box from direct sunlight will help out to some extent.

If this is florida, he is probably talking about central air and where the outdoor unit is. In most cases in Florida, the actual cooling coil is inside the house with the energy supply and fans outside the house. The air intake is usually also inside the house. Shading the external unit will do absolutely nothing.

In Florida, the usual issue if this wasn't happening in previous summers is either your AC unit is leaking or low on coolant, or your coil is dirty and slowly freezing up. Change your air filters regularly and call a maintenance man if you think this is something out of the normal.

If this is your first summer, be sure to white-blind all of your windows as often as possible (thick curtains with white panels on the outside where the sun would hit to reflect the light). Also be sure to check your doors and windows for leaks. Even the most powerful ac will likely not be able to keep up with the heat here, so seeing 74-77 if your ac is at 70 isn't always out of the norm if you don't have a well insulated house. Keep everything dark/closed as possible when you are out to keep insulation.

Nakatomi2010 · June 2010

Thanks for the info guys.

To clarify, the house is brand new, completed in October. The air conditioner are confident that it's more than enough to cool our house.

Also, the problem ONLY arises when the sun is shining on the side of the house that the air conditioner is on, not at noon, afternoon.

I'll look into some of the insulation tips you guys are posting, see if perhaps I can seal her up better.

I just don't believe that the sun shining hard on that A/C unit would make that kind of difference.

Also to clarify, there are only two windows on that side of the house, and both of the rooms those windows are in and COLD. It's primarily the main rooms of the house, kitchen, living room and a grand room. The gran room is empty.

Druhim · June 2010

Nakatomi2010 wrote: »

Thanks for the info guys.

To clarify, the house is brand new, completed in October. The air conditioner are confident that it's more than enough to cool our house.

Also, the problem ONLY arises when the sun is shining on the side of the house that the air conditioner is on, not at noon, afternoon.

I'll look into some of the insulation tips you guys are posting, see if perhaps I can seal her up better.

I just don't believe that the sun shining hard on that A/C unit would make that kind of difference.

Also to clarify, there are only two windows on that side of the house, and both of the rooms those windows are in and COLD. It's primarily the main rooms of the house, kitchen, living room and a grand room. The gran room is empty.

Yeah, but keep in mind that afternoon is the hottest part of the day barring atypical weather patterns. I think it's just sheer coincidence your unit is on that side of the house and it's simply an issue of it being the hottest time of day.

Midshipman · June 2010

Enc wrote: »

If this is florida, he is probably talking about central air and where the outdoor unit is. In most cases in Florida, the actual cooling coil is inside the house with the energy supply and fans outside the house. The air intake is usually also inside the house. Shading the external unit will do absolutely nothing.

Let me explain what you seem to be missing. The outside unit on central air houses the condenser coils along with a fan to provide airflow across the coils. The purpose of this part of the unit is to dump heat stored in the refrigerant (by changing it from a gas to liquid) so that the refrigerant is ready to head back inside and pick up more heat. This is roughly analogous to a radiator in a car. The hotter the environment that the condenser resides in, the harder the system has to work to dump out heat. Reducing the amount of radiant heating (sunshine) on the condenser box will ease the load on the system. It may or may not fix the problem that the OP is experiencing, but it will definitely save energy and contribute to the AC's effectiveness.

On another note, evening is usually when AC's have the most load on them regardless of where the condenser is located relative to shade. This is because while the sun and outside temperature both provide the most heating to the building between noon and shortly after, it takes a while to transfer that heat into the building. Attics, and to a lesser extent walls, are heat reservoirs that store up during the day. During the evening, they continue discharging heat into the home even as the sun falls away and the outside temperature starts to drop. Proper insulation and venting in the attic can go a long way to lessening the evening AC load.

Hawkstone · June 2010

It is also worth noting that sometimes the coils are located inside at your heater. in this case it may only be a matter of ensuring the loop out to the external fan is insulated properly. Also just to ensure that your AC guy is on the level, check your Filters to ensure they are clean and the proper size/ model for your unit this can empede air flow from the fan. I really do think it sounds like you are either in posession of an undersized fan or coil or the house is improperly insulated and your losing the cool air that the unit is building however.

bowen · June 2010

It is probably entirely unlikely the sun shining on the AC unit has any effect on it's umph.

The problem is thermodynamics and how hot and cold air work, and the fact that the warmest part of the day, like everyone else said, is in the afternoon. It's shitty, keep it cooler during the earlier parts of the day and see if that helps it stay cooler throughout the day rather than trying to force it colder once it's warmed.

Midshipman · June 2010

bowen wrote: »

It is probably entirely unlikely the sun shining on the AC unit has any effect on it's umph.

Sigh.

bowen · June 2010

Yes, this is one of those things that is primarily a coincidence, because of the time of day. The % decrease in output from the unit is probably negligible. The type of windows in the house has more of an effect on the temperature than external sunlight (edit: hitting the metal of an AC unit).

Spacemilk · June 2010

Uhh it's been years (4) since my thermo classes but I'm pretty sure Midshipman is right.

First of all, the coils won't be in the house because it would negate the effect of the A/C, right? It's that old thermo thought experiment - if you open the door to your fridge to cool your room, does it have any affect beyond the initial rush of cool air? The answer, once you work through all the calcs, is no. The coils on the back of the fridge have to heat up to provide cooling, and the radiant heat they produce will eventually negate the cool air coming out of the fridge. It negates it over a certain time scale, can't remember what, but since time scales don't apply to a permanently installed unit like A/C the point is - you would never have AC coils inside the house.

Therefore it stands to reason the coils are outside; and therefore any extra heat on the coils is extra load.

But bowen you may also be right because the extra load on the coils may be negligible next to the extra heat in the house from the sun. edit: yeah your post pretty much says what I was trying to say here, but better

Putting some sort of shade, or a white piece of cloth/tarp/cardboard/what have you may be enough to milk an extra degree or two out of your unit. But what's going to make a bigger difference is doing what GungHo said - check the insullation and other things. If you have lots of windows, install white blinds and keep them closed during the day.

bowen · June 2010

Yup, that's what I was trying to get at. I doubt that shading the unit will do anything unless it's specifically designed to keep radiant solar heat out. Depending on how you do it, you could make it worse, impacting air flow.

Spacemilk · June 2010

Also Nakatomi, I don't think you should give up about complaining about the A/C yet. The service manager's fix is going to have a big impact on your bills and (imo) isn't acceptable. I don't think you've said this yet but - is it a window unit or central air? Central air definitely should be able to handle it, and if it's a window unit and the house is newly built, then the unit is improperly sized for your house.

Basically I don't think you should give up on the A/C company yet. Still do everything else people have been saying in this thread, because it will all save you a lot on your energy bill, but keep bugging them about that A/C.

bowen · June 2010

It seems like an A/C unit from the way he's describing it. But that in no way should cool the entire house.

Spacemilk · June 2010

bowen wrote: »

It seems like an A/C unit from the way he's describing it. But that in no way should cool the entire house.

That's what I thought too, and it really depends on the size of the house. From his picture, it looks like a 3-story house with the a/c on the 2nd floor window, but (1) who would think it was a good idea to have one window unit on a 3-story house in Florida, and (2) never have I ever seen a window unit on the second story... but then I have never lived in Florida.

Anyway it's really hard to armchair diagnose the primary problem - the a/c unit - without more info.

neth · June 2010

I had an end unit townhome that had basically this same issue. during the evening/morning my house was cool, but during the day in direct sunlight I was roasting even with blinds/curtains blocking windows. They would come and check and the air coming out of the vents was always the proper amount less than the air in the house, it just wasn't getting cooler inside due to the facing of the building and the location of the a/c unit (direct sunlight). They shaded the A/C unit with some shrubs and that helped, so perhaps try that if you can.

edit: I was imagining his ascii drawing as a top down view showing the unit in the middle of the back of his house, perhaps i was mistaken

Nakatomi2010 · June 2010

My apologies.

The drawing is meant to be a one story home. The unit is on the west side of the house.

Nakatomi2010 · June 2010

In case anyone was wondering, the air conditioning people came out and replaced the entire unit, inside and outside, under warranty. So if the problem persists, I'll look into some shade.

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